Exemplary embodiments of the present invention relate to a navigation method for a missile, which comprises an infrared seeker head, an inertial navigation system (INS) and a device for the comparative evaluation of images just recorded with stored reference images. In accordance with exemplary embodiments an evaluation initially involves producing a multiplicity of three-dimensional (3D) lines for the three-dimensional reconstruction of the detected scene from the recorded images, and these lines are compared, taking into consideration the perspective of the reference available, and the current location and position of the missile is determined therefrom.
Various methods are known for the navigation of a missile. This includes first the use of a satellite navigation system (e.g., GPS—Global Positioning System), which allows a precise navigation only as long as the communication with the corresponding satellite is not disturbed.
Furthermore, inertial navigation systems are known, the precision of which, however, decreases with increasing flight distance. The combination of the two referenced systems does not result in the desired uninterrupted guidance along a specific flight route in every exemplary configuration.
It is therefore necessary to use a further system which processes image material recorded before the respective mission and provide it as a reference in the navigation computer of the missile. Reference structures of this type can be obtained from maps, satellite images or aerial photographs.
During the mission, for example, the seeker head takes over, which is directed from above at the flight path of the missile and continuously generates images, which are processed in the navigation computer of the missile and compared to the data of the aforementioned systems. It is customary to use infrared cameras for this purpose.
German patent document DE 10 2007 054 950 A1 discloses a method for supporting independent navigation of a missile having a forward looking camera and an inertial navigation system (INS), which compares the data from the current scene recorded by the camera to the reference images available in the missile. In particular, a multiplicity of three-dimensional lines is determined from the recorded images using a three-dimensional reconstruction. The multiplicity of three-dimensional lines are projected in the perspective of the available reference images. Subsequently, by aligning with the reference, the current position and location of the missile is determined and the navigation is supported. The alignment with an optical image is thus carried out. Therefore, it is sufficient to project the three-dimensional image generated from the camera image into a reference image. According to the object, this specification is specifically related to the use of a camera designed for the infrared range or for the visible range and the processing of the data thereof. There is no discussion of the use of systems that operate in other frequency ranges or to the special processing of the data of systems of this type.
To an increasing extent, reconnaissance systems operate using a radar with synthetic aperture (SAR). SAR sensors are powerful systems for long-distance reconnaissance. They are all-weather capable and provide a high spatial resolution, even for objects at a great distance. However, due to the special characteristics of this imaging principle, the interpretation of SAR images is difficult and very different from conventional image material.
German patent document DE 69306069 T2 describes a method that aligns a reference image to the SAR image of the missile in order to support navigation of the missile. This is achieved essentially through the correlation of the SAR image with the reference image. Ultimately, with this method only a 2D-2D alignment is carried out. With respect to the treatment of the known SAR artifacts, it is mentioned only that they lead to a slight deformation of the geometry of the image. In an oversimplification it is assumed that, despite this deformation, a correlation between the SAR image and the reference image leads to a correct localization of the missile and any errors in altitude that occur can subsequently be treated with methods of interference calculation. However, when the overflown scene has raised objects, such as high buildings, etc., the deformation of the geometry is no longer slight and a 2D-2D alignment will lead to imprecise results.
Exemplary embodiments of the present invention provide an expansion of known navigation systems for operation with reference images generated on an SAR basis, which is able to correctly resolve scenes with large differences in height.
This is achieved according to exemplary embodiments of the present invention by using SAR (Synthetic Aperture Radar) images as a reference, which originate from an SAR reconnaissance system at an earlier time, with a detected known trajectory and saved the data of the overflown terrain and from which reference images composed of three-dimensional line segments are generated, wherein together with the trajectory data associated with the respective image they are provided as a reference for the navigation method of the missile, and from the images of the infrared seeker head homogeneous areas bounded by three-dimensional lines and the spatial position thereof are obtained. Taking into consideration the three-dimensional structure of the overflown scene for each detected three-dimensional object, the corresponding azimuth position and range position are calculated, and the layover effects of the SAR images are modeled. Using analysis of the masking of three-dimensional areas, the SAR shadow edges are generated in the virtual reference edge image, and the virtual SAR images originating from the images of the infrared seeker head and generated from the calculations are compared to the reference edge images available as a reference and the trajectory data by means of a comparative evaluation, and continuously updated position data and location data of the missile are calculated therefrom.
Typical SAR artifacts are the reason that SAR images cannot simply be processed with the methods of the evaluation of an infrared image. An SAR image differs from an infrared image in that for each pixel the direction and the distance to the pixel, not location coordinates, are measured. As long as a scene is flat, this leads to images of equal quality, since in this case there is a clear connection between aspect angle and distance.
However, this situation changes when the scene is no longer flat or, for example, contains higher buildings. In this case, the SAR image generates, from the individual points of the building façade, a mirror image folded onto the ground, because these points lie at the same distance as their mirror image on the ground. This is referred to as the layover effect.
Another artifact is the so-called SAR shadow. This occurs when, due to the shielding of other objects in a certain distance range, no reflections occur. These SAR shadows often form clearly visible edges in SAR images, but have no counterpart in the optical image. These shadows mean that a direct alignment of SAR image and optical image leads to errors for scenes with raised objects.
The methods cited in the above-referenced patent documents, in which optical images and SAR images are compared directly with one another, consequently lead to a systematic positioning error.
The particular advantage of exemplary embodiments of the present invention can be seen in that for the first time a missile with infrared seeker head is using the image data of a high-precision SAR reconnaissance system as reference images, whereby it has been possible to clearly improve the precision of the navigation compared to previous systems.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.